EP3138814A1 - Method for the recovery of inorganic potassium salts from of an anaerobic and/or aerobic treatment of at least partially accessible substrates - Google Patents

Abstract

The present invention relates to a process for recovering inorganic potassium salts from at least one substrate which is at least partially accessible to anaerobic and / or aerobic treatment and comprising at least the steps (a), (b), (c), (d), (e) and ( f), namely a pre-cleaning comprising the steps (a), (b) and (c), ie separating solids contained in the substrate ((a)), removing nitrogen-containing compounds contained therein by means of stripping, wherein the pH of the substrate is adjusted to a pH> 9 prior to stripping by adding at least one alkaline inorganic potassium salt ((b)) and anaerobic and / or aerobic treatment of the substrate ((c)), wherein the order of (a), (b) and (c) is variable and after (a), (b) and (c ) a prepurified mixture (X) is obtained, treating (X) by nanofiltration to obtain a concentrate (D1) and a permeate (D2) ((d)), treating (D1) by electrodialysis to give a concentrate (E1) K) and a diluate (E1-D) and treating (D2) by electrodialysis to give a concentrate (E2-K) and a diluate (E2-D) ((e)), treating (E1-K) and ( E2-K) each independently by crystallization after at least partial concentration of the concentrates for recovery in each case in (E1-K) and (E2-K) contained inorganic potassium salts, which are different from the used in step (c) alkaline inorganic potassium salts ((f)), wherein the after step (e) available and in (E1 -K) and / or (E2-K) contained dissolved inorganic potassium salts and / or the respectively obtainable after step (f) inorganic potassium salts for the electrolytic production of the alkaline inorganic potassium salts used in step (b).

Description

The present invention relates to a process for recovering inorganic potassium salts from at least one substrate which is at least partially accessible to anaerobic and / or aerobic treatment and comprising at least the steps (a), (b), (c), (d), (e) and ( f), namely a pre-cleaning comprising the steps (a), (b) and (c), ie separating solids contained in the substrate ((a)), removing nitrogen-containing compounds contained therein by means of stripping, wherein the pH of the substrate is adjusted to a pH> 9 prior to stripping by adding at least one alkaline inorganic potassium salt ((b)) and anaerobic and / or aerobic treatment of the substrate ((c)), wherein the order of (a), (b) and (c) is variable and after (a), (b) and (c ) a prepurified mixture (X) is obtained, treating (X) by nanofiltration to obtain a concentrate (D1) and a permeate (D2) ((d)), treating (D1) by electrodialysis to give a concentrate (E1) K) and a diluate (E1-D) and treating (D2) by electrodialysis to give a concentrate (E2-K) and a diluate (E2-D) ((e)), treating (E1-K) and ( E2-K) each independently by crystallization after at least partial concentration of the concentrates for recovery in each case in (E1-K) and (E2-K) contained inorganic potassium salts, which are different from the alkaline inorganic potassium salts used in step (c) ((f)), whereby those obtainable after step (e) in (E1- K) and / or (E2-K) contained dissolved inorganic potassium salts and / or the respectively obtainable after step (f) inorganic potassium salts for the electrolytic production of the alkaline inorganic potassium salts used in step (b).

Anaerobic and / or aerobic treatment at least partially accessible substrates such as sewage sludge, in particular sewage sludge originating sewage sludge, liquid manure or residues from biogas plants are known in the art. Anaerobic treatment of such substrates, which results in anaerobic degradation by microorganisms, is for example fermentation. In particular, the focus here is on methane fermentation. In the Methane fermentation is formed by various interdependent microorganisms at the end of the degradation chain methane and carbon dioxide as major end products. The resulting gas mixture is also known as biogas and today has a significant industrial and economic relevance, especially in the energy production sector. For example, aerobic treatment of such substrates, resulting in, for example, aerobic degradation by microorganisms using oxygen, is composting or nitrification. Another example is the aerobic biological treatment of wastewater to form bacterial mass (excess sludge) and formation of carbon dioxide and water.

A process for obtaining cellulose or a mixture containing at least cellulose and at least lignocellulose from such fermentation, ie anaerobic treatment, of at least partially accessible substrate is disclosed in US Pat WO 2015/011275 A1 described.

Conventional methods for the purification or treatment of residues from biogas plants, manure or sewage treatment plants such as sewage sludge, i. from an anaerobic and / or aerobic treatment at least partially accessible substrates include a separation of the solids contained therein, for example by means of mechanical filtration, a biological purification by microorganisms (anaerobic and / or aerobic with separation of the biomass) and stripping in particular for the removal of contained therein nitrogen-containing compounds such as ammonia and amines, wherein the order of these steps may vary depending on the substrate used.

However, the prepurified mixtures obtained in this manner contain high salt loads, in particular high levels of inorganic salts, in particular inorganic alkali metal salts such as inorganic sodium salts, in particular sodium chloride, and / or potassium salts, in particular potassium chloride, potassium sulfate and / or aphthitalite (glaserite, K ₃ Na (SO ₄ ) ₂ ). Due to the presence of these salts in comparatively high proportions, the effluents produced during the work-up of, for example, raw sludge can not simply be fed into rivers or lakes, since the amounts of the salts contained therein exceed the statutory discharge limits. Even if an unrestricted introduction would be possible, the salts contained therein, in particular the inorganic alkali metal salts, would also be disposed of unused. This is undesirable from an economic point of view, in addition to sodium salts such as sodium chloride, which, for example, as industrial salt or as road salt for use in winter outdoor service, in particular potassium salts such as potassium chloride and potassium sulfate could be used for the production of fertilizers. It is therefore desirable if such inorganic alkali metal salts, in particular inorganic potassium salts, could be isolated and recycled from an anaerobic and / or aerobic treatment at least partially accessible substrates, such as sewage sludge, whereby at the same time the salt load of the effluents resulting from the treatment of sewage sludge, for example, would be lowered , which in turn would allow for easier disposal of these.

In conventional processes for the purification or treatment of residues from biogas plants or sewage treatment plants, the above-mentioned stripping, which in particular serves to remove nitrogen-containing compounds such as ammonia and amines contained therein, is usually carried out in an alkaline medium. In order to provide such an alkaline medium, alkaline inorganic sodium salts such as, in particular, sodium hydroxide are usually used recurrently, that is, continuously added in the case of a continuous process. The problem with this, however, is that the addition of such inorganic alkaline sodium salts in turn contributes to the overall increase in the salt load, which is undesirable for the reasons described in the preceding paragraph. The use of sodium hydroxide also has the disadvantage that in particular the concentration of sodium ions is increased and thereby the proportion of accumulating in the workup of sodium sulfate is increased: but this is also undesirable, since the possibilities of reusability of sodium sulfate compared to other inorganic alkali metal salts, in particular potassium salts such as potassium chloride, potassium sulfate and / or aphthitalite are substantially lower, in particular with regard to the amount of the resulting amount of sodium sulfate. However, the use of potassium hydroxide instead of sodium hydroxide has been used in practice undesirable because sodium hydroxide is much less expensive than potassium hydroxide. It is therefore desirable to provide a process for recovering inorganic potassium salts which not only allows direct recovery of the recovered potassium salts, but also allows these potassium salts to be converted into alkaline inorganic potassium salts, for example by electrolytic processes, such as, in particular, potassium hydroxide can then be recycled within the same preferably continuous process and, in particular, can be used as an inorganic base to adjust the pH during the stripping step: in this way an increase in the salt load could generally be avoided and in particular the amount of undesired sodium sulfate reduced and the cost disadvantage The use of alkaline inorganic potassium salts such as potassium hydroxide in comparison to sodium hydroxide could also by conversion of the isolated inorganic potassium salts and conversion of these in Be Ispiel potassium hydroxide be bypassed. In addition, costs could be saved, which are generated by the procurement and transport of commonly used alkaline inorganic sodium salts.

In addition to high levels of inorganic salts, however, even after carrying out the abovementioned customary steps such as solids separation, biological treatment and stripping, a number of compounds or components are often left in the mixture, which is then pre-cleaned in this way, which does not biodegrade or at least do not biodegrade sufficiently For example, (i) drugs derived from drugs and / or pesticides, (ii) humic acids or humic acid-like compounds, and (iii) so-called mucilages such as glycosaminoglycans (GAGs) or mucopolysaccharides. The presence of the gums can often be problematic when carrying out further purification steps by means of membrane processes such as ultra-, micro or nano-filtration, since these can deposit as membrane coatings on the membranes used in each case and thus reduce or impair the lifetime and effectiveness of the membranes used , The above-mentioned particular organic impurities (i) to (iii) should therefore be separated during the wastewater treatment at any time during the process. A possible separation represents the failure of at least the organic impurities (i) and (ii) in an acidic medium, for which purpose inorganic acids such as hydrochloric and / or sulfuric acid can be used or are, since these acids are relatively inexpensive commercially available. By using this acid and in particular the chloride or sulfate ions contained therein, however, the concentration of such anions and thus also the proportion of dissolved inorganic salts is increased overall. In other words, the use of hydrochloric and / or sulfuric acid also contributes to the precipitation of the salts Impurities (i) and (ii) to increase the total salt load at.

In addition, a pH-neutral mixture is usually used to carry out the ultra, micro or nano-filtration. On the other hand, since the step of stripping described above is usually carried out in an alkaline medium, the addition of acid such as inorganic acid, in particular hydrochloric and / or sulfuric acid, for neutralization is often necessary for this reason as well. Again, by using this acid and in particular the chloride or sulfate ions contained therein, the concentration of such anions and thus also the proportion of dissolved inorganic salts are increased overall. The use of hydrochloric and / or sulfuric acid to carry out the neutralization before carrying out the ultra, micro or nano-filtration thus also contributes to increasing the total salt load.

It is therefore desirable to provide a process for recovering inorganic potassium salts such as potassium chloride, potassium sulfate and aphthitalite, which not only provides direct recyclability of the recovered inorganic potassium salts but also conversion of these chloride and / or sulfate-containing potassium salts to inorganic salts Acids such as hydrochloric acid and sulfuric acid, which are then recycled within the same preferably continuous process and in particular for precipitation of the impurities (i) and (ii) can be used: in this way could increase the salt load by the continuous use of each new salt and or sulfuric acid are at least greatly reduced for the purpose of said precipitation.

There is therefore a need for an improved process which allows the targeted separation of inorganic potassium salts from the anaerobic and / or aerobic treatment at least partially accessible substrates such as sewage sludge from sewage sludge, residues from biogas plants and manure, on the one hand, the inorganic potassium salts thus obtained on the other hand, to be able to economically recycle and, on the other hand, to reduce the total salt load of the wastewater produced during wastewater treatment and, in particular, to at least largely free it from these salts, thus making it easier to dispose of this effluent. In particular, there is a need for such a method which does not have the disadvantages previously described in conventional prior art methods.

It is therefore an object of the present invention to provide a process for recovering inorganic potassium salts from at least partially accessible substrates from an anaerobic and / or aerobic treatment, such as sewage sludge originating from wastewater, residues from biogas plants and liquid manure. In particular, it is an object of the present invention to provide such a method which makes it possible, the inorganic potassium salts, from such anaerobic and / or aerobic treatment at least partially accessible substrates such as especially sewage-derived sewage sludge, residues from biogas plants and manure Separate, in order to be able to economically recycle them, for example as fertilizer, and at the same time also to reduce the salt load of wastewater treatment wastewater total and at least largely free these particular of these salts and thus allow easier disposal of this wastewater. In particular, it is an object of the present invention to obviate the drawbacks previously described in conventional prior art methods.

This object is solved by the subject matter claimed in the claims and the preferred embodiments of this subject disclosed hereinafter.

1. (a) Abtrennen von in dem wenigstens einen, einer anaeroben und/oder aeroben Behandlung zumindest teilweise zugänglichen Substrat enthaltenen Feststoffen,
2. (b) Entfernen von in dem wenigstens einen, einer anaeroben und/oder aeroben Behandlung zumindest teilweise zugänglichen Substrat enthaltenen stickstoffhaltigen Verbindungen mittels Strippen, wobei der pH-Wert des Substrats vor Durchführung des Strippens durch Zugabe wenigstens eines alkalischen anorganischen Kaliumsalzes auf einen pH-Wert > 9 eingestellt wird und
3. (c) anaerobe und/oder aerobe Behandlung des einer anaeroben und/oder aeroben Behandlung zumindest teilweise zugänglichen Substrats,
   wobei die Reihenfolge dieser Schritte (a), (b) und (c) variabel ist und nach Durchführung dieser Schritte (a), (b) und (c) eine vorgereinigte Mischung (X) erhalten wird,
4. (d) Behandeln der nach Durchführung der Vorreinigungsschritte (a), (b) und (c) erhaltenen vorgereinigten Mischung (X) mittels Nanofiltration unter Erhalt eines Konzentrats (D1) und eines Permeats (D2),
5. (e) Behandeln des nach Schritt (d) erhaltenen Konzentrats (D1) mittels Elektrodialyse unter Erhalt eines Konzentrats (E1-K) und eines Diluats (E1-D) und Behandeln des nach Schritt (d) erhaltenen Permeats (D2) mittels Elektrodialyse unter Erhalt eines Konzentrats (E2-K) und eines Diluats (E2-D), und
6. (f) Behandeln der nach Schritt (e) erhaltenen Konzentrate (E1-K) und (E2-K) nach zumindest teilweisem Einengen der Konzentrate jeweils unabhängig voneinander mittels Kristallisation, wobei Schritt (f) jeweils gegebenenfalls ein-oder mehrfach wiederholt wird, zur Gewinnung von jeweils in den Konzentraten (E1-K) und (E2-K) enthaltenen anorganischen Kaliumsalzen, die von den in Schritt (b) eingesetzten alkalischen anorganischen Kaliumsalzen verschieden sind,

dadurch gekennzeichnet, dass die nach Schritt (e) erhältlichen und in den Konzentraten (E1-K) und/oder (E2-K) enthaltenen gelösten anorganischen Kaliumsalze und/oder die nach Schritt (f) jeweils erhältlichen anorganischen Kaliumsalze zur elektrolytischen Herstellung von den in Schritt (b) verwendeten alkalischen anorganischen Kaliumsalzen eingesetzt werden.An object of the present invention is therefore a process for obtaining inorganic potassium salts from at least one at least partially anaerobic and / or aerobic treatment accessible substrate comprising at least the steps (a), (b), (c), (d), (e ) and (f), namely
a pre-purification comprising steps (a), (b) and (c):

1. (a) separating solids contained in the at least one substrate which is at least partially accessible to anaerobic and / or aerobic treatment,
2. (b) strip removal of nitrogen-containing compounds contained in the at least one substrate which is at least partially accessible to anaerobic and / or aerobic treatment, wherein the pH of the substrate before conducting the stripping by adding at least one alkaline inorganic potassium salt to a pH > 9 is set and
3. (c) anaerobic and / or aerobic treatment of the substrate which is at least partially accessible to an anaerobic and / or aerobic treatment,
   wherein the order of these steps (a), (b) and (c) is variable and after carrying out these steps (a), (b) and (c) a prepurified mixture (X) is obtained,
4. (d) treating the prepurified mixture (X) obtained after carrying out the prepurification steps (a), (b) and (c) by nanofiltration to obtain a concentrate (D1) and a permeate (D2),
5. (E) treating the obtained after step (d) concentrate (D1) by electrodialysis to obtain a concentrate (E1-K) and a diluate (E1-D) and treating the obtained after step (d) permeate (D2) by electrodialysis under Obtaining a concentrate (E2-K) and a diluate (E2-D), and
6. (f) treating the concentrates (E1-K) and (E2-K) obtained after step (e) after at least partial concentration of the concentrates in each case independently by crystallization, wherein step (f) is in each case optionally repeated one or more times, Recovery of each of the inorganic potassium salts contained in the concentrates (E1-K) and (E2-K), which are different from the alkaline inorganic potassium salts used in step (b),

characterized in that the obtained after step (e) and in the concentrates (E1-K) and / or (E2-K) dissolved inorganic potassium salts and / or after step (f) each available inorganic potassium salts for the electrolytic production of the used in step (b) alkaline inorganic potassium salts.

If the prepurified mixture (X) used in step (d) is neutralized before the nanofiltration is carried out, for example by setting a pH in the range of 6-8 or 7 ± 0.25, then at least one of these pH values will be used inorganic acid selected from the group consisting of hydrochloric acid, sulfuric acid and mixtures thereof. Preferably, in the concentrates (E1-K) and (E2-K) obtainable after step (e), chloride ions and / or sulfate ions are present, due to which the concentrates (E1-K) and / or (E2-K ) are preferably used for the electrolytic preparation of this used for neutralization hydrochloric acid, sulfuric acid and mixtures thereof.

It has surprisingly been found that the process according to the invention makes it possible to obtain inorganic potassium salts, in particular potassium chloride and aphthitalite, from at least one substrate which is at least partially accessible to anaerobic and / or aerobic treatment, in particular from sewage sludge, manure and / or residues from biogas plants, and these Salts thus for a variety of industries and for a variety of applications make usable, for example as fertilizers or for the production of fertilizers. In particular, it was surprisingly found that the process according to the invention makes it possible to obtain inorganic potassium salts, in particular potassium chloride, potassium sulfate and / or aphthitalite, in high yields and / or high degrees of purity, for example in a purity in the range from 75 to 95% by weight ,

In addition, it was surprisingly found that the process according to the invention makes it possible to separate off inorganic potassium salts, in particular potassium chloride, potassium sulfate and / or aphthitalite from inorganic sodium salts, such as sodium chloride, likewise contained in the substrates used, and in particular to make it possible for the inorganic alkali metal sulfate salts contained in the substrates Mainly as Aphthitalit K ₃ Na (SO ₄ ) ₂ to obtain, which can be used as fertilizer due to the only low Na content, which is only about 6.9 percent by weight, in contrast to sodium sulfate.

Furthermore, it has surprisingly been found that the inventive method not only allows the targeted separation of inorganic potassium salts from at least one of an anaerobic and / or aerobic treatment at least partially accessible substrate, but also the burden of inorganic alkali metal salts obtained in such a workup of such substrates wastewater ie their salt load, and thus facilitate the easier disposal of this effluent, for example by discharging it into waters such as rivers or lakes. In particular, the salinity of such wastewater can be reduced to values below the statutory discharge limits.

It has also been found, surprisingly, that the process according to the invention makes it possible for the inorganic potassium salts obtained by the process, ie in particular the solutions obtainable after step (e) and contained in the concentrates (E1-K) and / or (E2-K) inorganic potassium salts and / or the inorganic potassium salts respectively obtainable after step (f) for the electrolytic production of the alkaline inorganic potassium salts used in step (c), whereby the As a result, at least part of the recovered salts can be returned to the process cycle, especially if the process according to the invention is a continuous process. Alkaline inorganic potassium salts (or other alkaline inorganic salts such as alkaline inorganic sodium salts required to carry out step (c) need not be added separately - or at least added only once - and thus do not contribute to salt stress the wastewater, especially if the inventive method is a continuous process.

Finally, it was surprisingly found that the inventive method not only allows direct recovery of the recovered potassium salts such as potassium chloride and Aphthitalit, but also allows a conversion of these chloride and / or sulfate-containing potassium salts into inorganic acids such as hydrochloric acid and sulfuric acid, which then within can be recycled to the same preferably continuous process and in particular for precipitation of the impurities mentioned in the introductory part (i) and (ii) can be used: in this way can increase the salt load by a continuous use of each new hydrochloric and / or sulfuric acid to said Precipitation be avoided.

In addition, it has surprisingly been found that - if the pre-purified mixture (X) used in step (d) is neutralized by carrying out the nanofiltration by adding at least one inorganic acid selected from the group consisting of hydrochloric acid, sulfuric acid and mixtures thereof, for example by adjusting a pH in the range of 6-8 or 7 ± 0.25, - the aforementioned at least one inorganic acid to adjust this pH due to the presence of chloride ions and / or sulfate ions in the after step (e) available Concentrates (E1-K) and / or (E2-K) can be produced electrolytically from (E1-K) and (E2-K), thus avoiding the continual addition of new acid and thus also an increase in the salt load can be avoided.

substratum

The inventive method allows the recovery of inorganic potassium salts from at least one, an anaerobic and / or aerobic treatment at least partially accessible substrate.

As an anaerobic and / or aerobic treatment at least partially accessible substrates, in particular all the skilled person in connection with the fermentation as an example of an anaerobic treatment, in particular methane fermentation, and the associated biogas production known substrates can be used. These include in particular all substrates containing rotatable, or fermentable and / or aerobically degradable, organic compounds. As fermentation known to be the anaerobic degradation of organic compounds and components by microorganisms, ie the decomposition of more complex, optionally polymeric organic compounds or components such as polysaccharides, fats and proteins in low molecular weight compounds such as methane, carbon dioxide and water and ammonia and hydrogen sulfide.

The at least one substrate which is at least partially accessible to anaerobic and / or aerobic treatment is preferably present in the form of an aqueous mixture.

Preferably, the at least one at least partially anaerobic and / or aerobic treatment substrate selected from the group consisting of grass, corn, grass silage, corn silage, Dünnschlempe, feed beet, whole plant silage, green waste, stalk biomass, industrial fruit, manure, press cake, rapeseed meal, rapeseed cake Beet leaf silage, sugar beet, manure, biowaste and sewage sludge, preferably raw sludge, in particular sewage or waste water or from residues of biogas plants derived sewage sludge, preferably raw sludge. For the purposes of this invention, "manure" is preferably understood as meaning fertilizer such as farm manure, particularly preferably naturally occurring manure such as farm manure, which contains at least urine and feces of agricultural livestock. Depending on the amount of bedding contained in it, the term "manure" includes thick or thin manure, Flourmist or liquid manure. In the context of the method according to the invention, at least one substrate which is at least partially accessible to anaerobic and / or aerobic treatment is particularly preferably used a substrate which is selected from the group consisting of sewage sludge, manure, residues from biogas plants (so-called fermentation residues) and mixtures thereof.

Inventive method

The process according to the invention is preferably a process for obtaining inorganic potassium salts selected from the group comprising at least potassium chloride, potassium sulfate and / or aphthitalite (glaserite; K ₃ Na (SO ₄ ) ₂ ), more preferably selected from the group comprising at least potassium chloride and / or Aphthitalit.

In addition to the production of inorganic potassium salts, the process according to the invention preferably also additionally permits the recovery of inorganic sodium salts, in particular sodium chloride. These inorganic sodium salts can also be obtained by crystallization according to step (f) of the process according to the invention from the concentrates (E1-K) and / or (E2-K) obtained according to step (e), in particular (E2-K).

Preferably, the process of the invention is a continuous process.

The process according to the invention comprises three pre-purification steps, namely steps (a), (b) and (c), wherein the order of these steps (a), (b) and (c) is variable and after all these steps (a) have been carried out , (b) and (c) a pre-purified mixture (X) is obtained, which is then used in the subsequent step (d).

1. (i) Durchführung der Schritte (a), (b) und (c) nacheinander in dieser Reihenfolge. Die in Schritt (d) eingesetzte vorgereinigte Mischung (X) entspricht dann der nach Schritt (C) erhaltenen Mischung. Diese Reihenfolge ist insbesondere dann bevorzugt, wenn als Substrat Klärschlamm, vorzugsweise Rohschlamm, insbesondere aus Abwasser oder Abwässern stammender Klärschlamm, oder Gülle eingesetzt wird.
2. (ii) Durchführung der Schritte (c), (a) und (b) nacheinander in dieser Reihenfolge. Die in Schritt (d) eingesetzte vorgereinigte Mischung (X) entspricht dann der nach Schritt (B) erhaltenen Mischung. Diese Reihenfolge ist insbesondere dann bevorzugt, wenn als Substrat Rückstände aus Biogasanlagen eingesetzt wird.
3. (iii) Durchführung der Schritte (a), (c) und (b) nacheinander in dieser Reihenfolge. Die in Schritt (d) eingesetzte vorgereinigte Mischung (X) entspricht dann ebenfalls wie nach Variante (ii) der nach Schritt (B) erhaltenen Mischung.

Since the sequence of steps (a), (b) and (c) is variable, in particular the following variants of the sequence of these pre-purification steps are included and preferred:

1. (i) performing steps (a), (b) and (c) sequentially in this order. The prepurified mixture (X) used in step (d) then corresponds to the mixture obtained after step (C). This sequence is particularly preferred when the substrate used is sewage sludge, preferably raw sludge, in particular sewage sludge originating from sewage or effluents, or manure.
2. (ii) performing steps (c), (a) and (b) sequentially in this order. The prepurified mixture (X) used in step (d) then corresponds to the mixture obtained after step (B). This sequence is particularly preferred when residues from biogas plants are used as the substrate.
3. (iii) performing steps (a), (c) and (b) sequentially in this order. The pre-purified mixture (X) used in step (d) then also corresponds to the mixture obtained after step (B) as in variant (ii).

Step (a)

Step (a) of the pre-purification within the method according to the invention comprises a separation of solids contained in the at least one substrate which is at least partially accessible to the fermentation to obtain a mixture (A). This mixture (A) can be used as a prepurified mixture (X) in step (d), provided that step (a) is carried out only after step (b) and (c).

The separation of the solids in step (a) preferably takes place mechanically, more preferably in the form of a mechanical separation. For this purpose, for example, a separation by means of rakes and / or sand traps is suitable. In the context of this separation, the mechanical separation method known to the person skilled in the art can be used. These separation processes are, in particular, sedimentation, centrifuging and filtration or sieve filtration, optionally in combination with presses (application of filter presses). Such separation processes can be carried out, for example, by using conventional equipment and devices, as are known to the person skilled in the art. Within many of these systems and devices, several of the separation processes or separation principles mentioned are interrelated combined. Applicable systems are, for example, vacuum belt filters. Corresponding screening and filtration systems and a corresponding selection of sieves and / or filters can be easily selected by the skilled person. Also by ultrafiltration and / or microfiltration separation of the solids according to step (a) can be carried out. This sieve or sieve filtration step can preferably be supplemented by an additional flushing of the sieve residue with water. The separation of the solids according to step (a) can also be carried out by means of helical filters and / or sand filters and by sedimentation in sedimentation tanks. An overview of usable separation methods and separation principles can be found in the general technical literature, for example " Technical Microbiology "by Hermann Kretzschmar, pages 265 to 286, (Parey, 1968, ISBN 3489715144 ) or the manual " Filters and Filtration Handbook "(Elsevier, Advanced Technology, 4th Edition, ISBN 185617322 ).

For example, filter presses, rotary filter systems, screen drum presses, belt presses, screw extruders, centrifuges of various sizes and types such as decanters (screw discharge centrifuges), Siebkerbbeziehweise filter centrifuges, tube centrifuges, multi-chamber centrifuges or nozzle separators, microfiltration systems, ultrafiltration systems, filters such as Sandilfter and / or helical filters and Vacuum belt filter can be used with screen inserts.

Preferably, step (a) of the process according to the invention represents at least one separation process selected from the group of sieve filtration and centrifugation processes and ultrafiltration processes and / or microfiltration processes and processes which combine the abovementioned processes with one another. The separation in step (a) is preferably carried out in the form thus in a mechanical separation, more preferably by means of a sieve filtration, centrifugation or a combined Siebfiltrationszentrifugation. In the combination of the methods, the order of use of the method depends essentially on the nature of the mixture to be separated, in particular its fiber content and / or its gel content. Prefers For this purpose press screw separators and / or decanters (screw discharge centrifuges) are used.

Step (b)

Step (b) of the prepurification within the method according to the invention comprises stripping by removal of nitrogen-containing compounds contained in the at least one substrate which is at least partially accessible to anaerobic and / or aerobic treatment, wherein the pH of the substrate before stripping by addition of at least of an alkaline inorganic potassium salt is adjusted to a pH> 9 to obtain a mixture (B). This mixture (B) can be used as a prepurified mixture (X) in step (d), provided that step (b) is carried out only after steps (a) and (c).

The alkaline inorganic potassium salts used are different from the inorganic potassium salts obtainable after step (e) and contained in the concentrates (E1-K) and (E2-K) and / or from the inorganic potassium salts obtainable after step (f).

The term "nitrogen-containing" compounds are understood to mean in particular organic nitrogen-containing compounds such as amines and / or ammonium salts and / or ammonia. Preferably, these nitrogenous compounds are not understood to be inorganic nitrate salts, i. in particular, no nitrogen-containing potassium and / or sodium nitrate salts such as potassium nitrate.

Preferably, the alkaline inorganic potassium salts used in step (b) are selected from the group consisting of potassium hydroxide, potassium carbonate, potassium bicarbonate and mixtures thereof. Particularly preferred is potassium hydroxide.

The person skilled in the art is familiar with the term "stripping". When stripping (= expelling) are preferably from liquids (preferably water in wastewater treatment) ingredients of these liquids by the passage of gases (air, water vapor, flue gas, etc.) removed from the liquid and in the Gas transferred. The driving force behind this process is that the vapor pressure of the substances to be removed from the liquid in the liquid is greater than in the gas and therefore a transfer from the liquid to the gas takes place. The liquid is preferably passed countercurrently, ie counter to the flow direction of the gas, through a so-called stripping column (a tubular device with turbulence-inducing internals).

Step (c)

Step (c) of the pre-purification within the method according to the invention comprises an anaerobic and / or aerobic treatment of the substrate which is at least partially accessible to an anaerobic and / or aerobic treatment to obtain a mixture (C). This mixture (C) can be used as a prepurified mixture (X) in step (d), provided that step (c) is carried out only after steps (a) and (b). Part of the anaerobic and / or aerobic treatment according to step (c) is preferably the carrying out of an ultrafiltration. The resulting ultrafiltration permeate is preferably used as mixture (C), which in turn can be used as prepurified mixture (X) in step (d).

Anaerobic treatment is a biological process carried out in the presence of microorganisms. For example, step (c) provides for controlled anaerobic digestion or fermentation carried out in digester towers. The slightly rotting compounds are decomposed by anaerobic bacteria cultures gradually. By means of a first phase, the so-called hydrolysis and acidification phase, the substances are converted by corresponding microorganisms mainly to short-chain fatty acid or carboxylic acids and alcohols as well as hydrogen and carbon dioxide. The acetogenic phase ultimately produces acetic acid, which is then converted in the second stage, the methane phase, by corresponding microorganisms (methane bacteria) to methane and carbon dioxide. In addition, the conversion of hydrogen and carbon dioxide to methane and water is effected. In addition to the main product methane, which is deliberately dissipated and then used as a biogas component or for general power generation (grid feed), in the digester, for example, nitrogen-containing compounds such as in particular ammonia or corresponding ammonium salts arise, which can be removed according to step (b). The after about 20 to 30-day digestion process remaining so-called digested sludge (stabilized sewage sludge) contains a high amount of inorganic compounds such as inorganic alkali metal salts, which are not converted or degraded in the digester or only to a small extent. Often, even larger solid particles and agglomerates present in the original raw sludge are only partially decomposed, even though they contain the anaerobically decomposable materials. The reason for this is that the attack surface for the anaerobic conversion in such agglomerates is correspondingly small.

The aerobic treatment is a biological process which is carried out in the presence of microorganisms under the action of oxygen, in particular atmospheric oxygen.

Step (d)

Step (d) of the process according to the invention comprises a treatment of the prepurified mixture (X) obtained by carrying out the prepurification steps (a), (b) and (c) by nanofiltration to obtain a concentrate (D1) and a permeate ( D2).

The pre-purified mixture (X) is preferably subjected to microfiltration and / or ultrafiltration before carrying out step (d), in particular if step (a) does not represent such microfiltration and / or ultrafiltration. In particular, in this case, the respective concentrate obtained in microfiltration and / or ultrafiltration is used as prepurified mixture (X) in step (d).

Preferably, the concentrate (D1) obtained after carrying out step (d) has a higher amount of sulfate ions compared to the mixture (C) used in step (d) and the permeate (D2) obtained in step (d) Compared to the concentrate (D1) obtained in step (d), a higher amount of chloride ions.

The term "nanofiltration" is known to the person skilled in the art, for example from the textbook " Membrane method "by Thomas Melin and Robert Rautenbach, Springer Verlag, 2nd edition, 2004 (Item 10 .: Nanofiltrati on).

Preferably, step (d) of the process according to the invention is carried out at the temperature which comprises the prepurified mixture (X) after carrying out the steps (a), (b) and (c). In particular, step (d) does not involve cooling the prepurified mixture (X). Preferably, the prepurified mixture (X) has a temperature in the range from 5 to 60 ° C or from 5 to 58 ° C, particularly preferably from 10 to 50 ° C or from 20 to 50 ° C, in particular from 20 to 45 ° C, most preferably from 20 to 35 ° C, at which step (d) is carried out accordingly. Preferably, step (d) is carried out at a pH of the prepurified mixture of 6 to 8, more preferably at pH 7 ± 0.25.

In step (d) of the method according to the invention, a nanofiltration membrane is used to carry out the nanofiltration. Any commercially available nanofiltration membrane (NF membrane) is suitable for this purpose. Preferably, such commercially available NF membranes are substantially permeable to monovalent ions such as chloride ions, but are substantially impermeable to divalent ions such as sulfate ions. Preferably, divalent ions such as sulfate ions are retained by at least 80%, particularly preferably at least 90%, in each case based on the total amount of the sulfate ions contained in the mixture (X) by the NF membrane used. Preferably, monovalent ions such as chloride ions are transmitted to at least 50 or at least 55%, based in each case on the total amount of the chloride ions contained in the mixture (X) through the NF membrane used. The so-called "separation limit" of the NF membrane used is preferably <200 daltons.

Preferably, in step (d) at least partially an NF membrane is used which is based on polysulfones as a carrier material, which is preferably at least partially coated, more preferably at least partially with at least one polyamide homo- or copolymers. In addition, if necessary, other conventional NF membranes can also be used.

Step (d) is preferably carried out within a nanofiltration unit consisting of three units per module, ie per pressure tube. Preferably, each unit is about 1 m long and has a diameter of about 20 cm. Each unit preferably has an effective area of from 30 to 40 m ² , preferably from 34 to 38 m ² . Preferably, the aforementioned organic impurities (iii) are separated on the NF membrane, ie they deposit on the NF membrane.

Preferably, step (d) takes place in the so-called crossflow mode, which provides for a comparatively high overflow of the NF membrane in order to counteract a possible concentration polarization.

In carrying out step (d), the so-called Donnan effect can be exploited. This is known to the person skilled in the art (cf. Membrane Process "by Thomas Melin and Robert Rautenbach, Springer Verlag, 2nd edition, 2004 (Item 10.4: Donnan Effect).

The permeate (D2) obtainable after step (d) of the process according to the invention preferably contains no organic impurities, in particular no organic impurities selected from the group consisting of (i) active substances from medicaments or pesticides, (ii) humic acids and / or humic acid-like compounds and ( iii) mucilages such as glycosaminoglycans (GAG) and / or mucopolysaccharides.

Preferably, before carrying out step (d), preferably by adding at least one inorganic acid to the prepurified mixture (X) obtained after carrying out the prepurification steps (a), (b) and (c) - in any order - a pH in the range from 3 to 8, or 4 to 8 or 6 to 8, in particular 5 to 7, particularly preferably adjusted from pH 7 ± 0.25. Preferably, the inorganic acid used for this purpose is selected from the group consisting of hydrochloric acid, sulfuric acid and mixtures thereof. The dissolved inorganic potassium salts which are obtainable after step (e) and contained in the concentrates (E1-K) and (E2-K) and / or the inorganic potassium salts obtainable after step (f) are preferably added to the electrolytic catalyst Preparation of the inorganic acids used in this step used. This is possible due to the chloride and sulfate ions contained in (E1-K) and (E2-K).

Optional step (d-1)

• (d-1) Behandeln des Permeats (D2) mittels einer Membranfiltrationstechnik, vorzugsweise mittels Umkehrosmose oder Nanofiltration, besonders bevorzugt mittels Umkehrosmose, unter Erhalt eines Konzentrats (D2-K) und eines Permeats (D2-P), wobei das Konzentrat (D2-K) in diesem Fall im nachfolgenden Schritt (e) anstelle des Permeats (D2) eingesetzt wird.

Preferably, the permeate (D2) obtained after carrying out step (d) is subjected to a step (d-1) before carrying out the electrodialysis according to step (e), which will be described in more detail below

• (d-1) treating the permeate (D2) by means of a membrane filtration technique, preferably by means of reverse osmosis or nanofiltration, particularly preferably by reverse osmosis, to obtain a concentrate (D2-K) and a permeate (D2-P), the concentrate (D2-P) K) is used in this case in the subsequent step (e) instead of the permeate (D2).

The term "reverse osmosis" is known to the person skilled in the art. It is a process step for the concentration of substances dissolved in liquids or mixtures in which the natural osmosis process is reversed by the action of pressure. In this case, the medium in which the concentration of a certain component is to be reduced, separated by a semi-permeable (semipermeable) membrane from the medium in which the concentration is to be increased. This is subjected to pressure that must be higher than the pressure created by the osmotic desire to balance the concentration. This allows the molecules of the solvent to migrate towards their "natural" osmotic propagation direction. The process pushes them into the compartment where dissolved components are less concentrated.

Preferably, after performing step (d) and before performing step (e), the permeate (D2) obtained after step (d) is further concentrated in such a step (d-1). This can be done by means of a suitable membrane process such as nanofiltration or reverse osmosis. Preferably, the permeate (D2) is treated by reverse osmosis, after which such a reverse osmosis a concentrate (D2-K) and a permeate (D2-P) are obtained. If such a step (d-1) is performed, preferably the concentrate (D2-K) thus obtained is used instead of the permeate (D2) in step (e) of the process according to the invention. The available permeate (D2-P) can be discarded, or can be introduced as low-salt water.

Steps)

Step (e) of the process of the present invention comprises treating the concentrate (D1) obtained by step (d) by electrodialysis to obtain a concentrate (E1-K) and a diluate (E1-D) and treating it after step (d) Permeate (D2) by electrodialysis to obtain a concentrate (E2-K) and a diluate (E2-D).

The term "electrodialysis" is known to the person skilled in the art, for example from the textbook " Handbuch der Abwasser- und Recyclingtechnik "by Ludwig Hartinger, Carl Hanser Verlag, 2nd edition, 1991 (Item 7.6.3 on pages 523-532 ). The construction and operation of electrodialysis systems is known in the art, for example from the textbook " Membrane method "by Thomas Melin and Robert Rautenbach, Springer Verlag, 2nd edition, 2004 (Item 12.3 on pages 367-374 ).

Preferably, in the concentrate (E1-K) obtained after step (e), essentially sulfate ions as anions and potassium ions (and sodium ions) as cations are contained. Preferably, the concentrate (E1-K) obtained after step (e) is depleted in dissolved sodium chloride, ie it contains a substantially smaller amount of dissolved sodium chloride than the concentrate (E2-K) also obtained after step (e). The sulfate ions and potassium ions (as well as sodium ions) contained in the concentrate (E1-K) can be obtained in the following step (f) in the form of aphthitalite (K ₃ Na (SO ₄ ) ₂ ). From mixtures in which sulfate ions and potassium ions and sodium ions are present, it is usual to obtain predominantly aphthitalite (K ₃ Na (SO ₄ ) ₂ ), ie a mixed double sulphate salt and not pure potassium sulphate and / or sodium sulphate (cf. U.S. Patent No. 1,936,070 ). Small amounts of potassium sulfate may be included therein.

Preferably, in the concentrate (E2-K) obtained after step (e), essentially chloride ions are used as anions and potassium ions (and sodium ions) as cations contain. Preferably, the concentrate (E2-K) obtained after step (e) is rich in chloride ions, ie it contains a significantly higher amount of dissolved sodium chloride than the concentrate (E1-K) also obtained after step (e). The chloride ions and potassium ions contained in the concentrate (E2-K) can be obtained in the following step (f) in the form of potassium chloride (KCl). In this case, a separation of KCl can be carried out by means of the sylvinite method (cf. Textbook of Chemical Technology, Winnacker and Kuchler, Volume 2, Inorganic Technology 1, Carl Hanser Verlag, 4th edition, 1982, chapter 3.34, pages 288 to 294 ).

The term "essentially" in connection with the sulphate ions contained in the concentrate (E1-K) as anions as well as potassium ions (and sodium ions) as cations in the context of the present invention preferably means that at least 70% by weight. , preferably at least 80% by weight, particularly preferably at least 85% by weight, very particularly preferably at least 90% by weight, which are sulfate ions in the concentrate (E1-K) or at least 70% by weight , Preferably at least 80 wt .-%, more preferably at least 85 wt .-%, most preferably at least 90 wt .-%, in the concentrate (E1-K) cations are potassium ions (and sodium ions).

The term "substantially" in connection with the chloride ions contained in the concentrate (E2-K) as anions as well as potassium ions (and sodium ions) as cations preferably means within the meaning of the present invention that at least 70% by weight , preferably at least 80% by weight, more preferably at least 85% by weight, most preferably at least 90% by weight, of anions are chloride ions in the concentrate (E2-K) or at least 70% by weight , Preferably at least 80 wt .-%, more preferably at least 85 wt .-%, most preferably at least 90 wt .-%, in the concentrate (E2-K) cations are potassium ions (and sodium ions).

In particular, the concentrate (E2-K) preferably also contains dissolved inorganic sodium salts, in particular sodium chloride, in addition to the dissolved inorganic potassium salts contained therein. Preferably also contains the concentrate (E1-K) in addition to the dissolved inorganic contained therein Potassium salts also dissolved inorganic sodium salts, especially sodium chloride, but the amount of dissolved inorganic sodium salts contained therein is smaller than in the case of concentrate (E2-K).

The diluate (E1-D) obtainable after step (e) of the process according to the invention comprises, after carrying out the electrodialysis according to step (e) - if present in the substrate originally used - organic impurities, in particular organic impurities selected from the group consisting of (i) active substances from drugs or pesticides and (ii) humic acids and / or humic acid-like compounds. These may optionally be separated in optional step (h) described below, preferably by acid precipitation to give the filtrate (E1-D-K-F).

Step (f)

Step (f) of the process according to the invention comprises treating the concentrates (E1-K) and (E2-K) obtained after step (e) after at least partial concentration of the concentrates in each case independently of one another by means of crystallization, step (f) in each case optionally or repeated several times, for the recovery of each of the concentrates (E1-K) and (E2-K) dissolved inorganic potassium salts, which are different from the used in step (c) alkaline inorganic potassium salts.

Preferably, the crystallization according to step (f) in each case comprises at least three stages (f1), (f2) and (f3).

Stage (f1) looks

• (f1) concentrating the concentrates (E1-K) and (E2-K) obtained after step (e) each independently at a temperature in the range from 75 ° C to the boiling point of the concentrates (E1-K) and (E2-K ), wherein in (E2-K) contained inorganic sodium salt, in particular sodium chloride, during the implementation of (f1) at least partially precipitates at this temperature and in (E1-K) contained inorganic sodium salt, especially sodium chloride, during the implementation of (f1 ) At this temperature, if appropriate, at least partially precipitated, and wherein the step (f1) is carried out until the respective contained therein until this time dissolved inorganic potassium salts begin to precipitate at this temperature.

Due to the inorganic salts present in the concentrates (E1-K) and (E2-K), these concentrates have a boiling point> 100 ° C., for example a boiling point of> 100 to 115 ° C. The boiling point refers to the boiling point at atmospheric pressure, i. at 1 bar.

If it is observed during the implementation of step (f1), in particular starting from (E2-K), that - in addition to inorganic sodium salts, in particular sodium chloride, - the dissolved inorganic potassium salts present therein in each case begin to precipitate, it is preferred to immediately go to step ( f2).

Methods are known to the person skilled in the art by means of which it can be determined at which time during the performance of stage (f1) the dissolved inorganic potassium salts present in (E1-K) and (E2-K) in each case begin to precipitate. This time can be determined by inductively coupled plasma atomic emission spectrometry (ICP-OES) according to DIN EN ISO 11885 (date: 1.9.2009), wherein the content of the elements sodium or potassium of a sample to be examined, ie a sample containing the precipitated salts during the performance of step (f1). This sample or a solution of the respective sample is thermally excited in an argon plasma generated by a high frequency field and the light emitted due to electron transitions visible as a spectral line of the corresponding wavelength and analyzed with an optical system. There is a linear relationship between the intensity of the emitted light and the presence or concentration of the corresponding element, such as sodium or potassium. Before carrying out the test, calibration measurements are carried out on the basis of known element standards (reference standards), which are dependent on the particular sample to be examined. These calibrations can be used to determine the presence (or non-presence) of potassium in the sample.

Stage (f2) looks

Filtering off the inorganic sodium salts, in particular sodium chloride, which have at least partly precipitated after carrying out step (f1) from (E2-K) and, if appropriate, filtering off the inorganic sodium salts, in particular sodium chloride, which may optionally have been at least partially precipitated after carrying out step (f1) from (E1-K) , at the aforementioned temperature according to step (f1) each independently to obtain the at least partially precipitated from inorganic salts (E2-K), in particular sodium chloride, and the filtrate (E2-KF) and the concentrated concentrate (E1-K) or the from (E1-K) at least partially precipitated inorganic sodium salts, in particular sodium chloride, and the filtrate (E1-KF), before.

The at least partially precipitated salts during the performance of (f1) from (E2-K) essentially contain sodium chloride. Preferably, the inorganic salts filtered off according to step (f2), in particular the inorganic salts precipitated from (E2-K), comprise sodium chloride in an amount of at least 85% by weight, more preferably of at least 90% by weight, in particular of at least 95% by weight .-%, in each case based on the total weight of the filtered inorganic salts. When step (f1) is carried out, inorganic sodium salts can be precipitated from the concentrate used (E1-K). In this case, they are filtered off within the step (f2) and the filtrate (E1-K-F) is obtained. If no inorganic sodium salts precipitate when step (f1) is carried out, step (f2) is omitted in this case and the concentrate (E1-K) concentrated according to step (f1) is used directly in step (f3).

Stage (f3) looks

Cooling of the filtrate (E2-KF) and filtering off then precipitated inorganic potassium salts and concentration of the filtrate (E1-KF) or further concentration of the already concentrated concentrate (E1-K), each with the then from (E1-KF) or from the already concentrated concentrate (E1-K) precipitated inorganic potassium salts, before.

The cooling according to step (f3) in connection with the cooling of the filtrate (E2-K-F) is preferably carried out at a temperature <35 ° C, more preferably <30 ° C. Therefore, before or during the performance of step (f3), it is preferable to cool to such a temperature.

The concentration of the filtrate (E1-KF) or further concentration of the (already) concentrated concentrate (E1-K) takes place in step (f3) to obtain the then from (E1-KF) or from the already concentrated concentrate (E1-K) precipitated inorganic potassium salts. Preferably, this concentration is carried out at a temperature in the range of 75 ° C to the boiling point of the concentrated concentrate (E1-K) or the filtrate (E1-K-F).

Preferably, after step (f3) from (E1-KF) or from the concentrated concentrate (E1-K), aphthitalite is obtained and from (E2-KF) potassium chloride is obtained, preferably each independently in a purity in the range of 70 to 95 wt. -%.

When carrying out steps (f1) to (f3) of step (f) of the process according to the invention and obtaining potassium chloride from (E2-KF), it is separated off by means of the sylvinite process: in this case, after carrying out step (f2) take advantage of the fact that sodium chloride dissolves in the coexistence of potassium chloride as a sediment at higher temperature worse than at low temperature: this remains in (E2-KF) sodium chloride contained in the cooling according to step (f3) in solution, whereas potassium chloride precipitates.

Preparation of Alkaline Inorganic Potassium Salts Used in Step (b)

The dissolved inorganic potassium salts obtainable after step (e) and contained in the concentrates (E1-K) and / or (E2-K) and / or the inorganic potassium salts obtainable after step (f) are used for the electrolytic production of b) used alkaline inorganic potassium salts.

Preferably, this electrolytic production is a membrane electrolysis, particularly preferably a membrane electrolysis, in which a four-chamber electrolysis cell is used. In this way it can be ensured, in particular, that the chloride ions of the dissolved inorganic potassium salts contained in the concentrates (E1-K) and / or (E2-K) and / or the inorganic potassium salts which are respectively obtainable after step (f) do not become chlorine be implemented.

In addition, inorganic acids, such as hydrochloric acid and / or sulfuric acid, which can be used to adjust an acidic pH, for example for adjusting an acidic pH, are preferably produced in the electrolytic production of the alkaline inorganic potassium salts used in step (b) Step (c) mixture (C) obtained or the concentrate (D1) obtained after step (d) and / or the permeate (D2) obtained after step (d), should this be necessary.

Optional steps (g), (h) and (i)

• (g) Behandeln des nach Schritt (e) erhaltenen Diluats (E1-D) mittels Umkehrosmose oder Nanofiltration unter Erhalt des Konzentrats (E1-D-K) und des Permeats (E1-D-P),
• (h) Einstellen des pH-Werts des Konzentrate (E1-D-K) durch Zugabe wenigstens einer anorganischen Säure auf einen pH-Wert ≤4 und Abfiltrieren der dann jeweils ausgefallenen Feststoffe unter Erhalt des Filtrats (E1-D-K-F) und
• (i) Zuführen des nach Schritt (h) erhaltenen Filtrate (E1-D-K-F) zu dem nach Schritt (d) erhaltenen Konzentrat (D1).

Preferably, the method according to the invention comprises at least the further steps (g), (h) and (i), namely

• (g) treating the diluate (E1-D) obtained after step (e) by means of reverse osmosis or nanofiltration to obtain the concentrate (E1-DK) and the permeate (E1-DP),
• (h) adjusting the pH of the concentrate (E1-DK) by adding at least one inorganic acid to a pH ≤4 and filtering off the then precipitated solids to obtain the filtrate (E1-DKF) and
• (i) feeding the filtrate (E1-DKF) obtained after step (h) to the concentrate (D1) obtained after step (d).

The diluate (E2-D) obtained after step (e) can be discarded or derived. The same applies to the permeate (E1-D-P) obtained after step (g).

Preferably, the inorganic acid used in step (h) is selected from the group consisting of hydrochloric acid, sulfuric acid and mixtures thereof, and those obtainable after step (e) and in the concentrates (E1-K) and / or (E2-K) contained inorganic potassium salts and / or the respectively obtainable after step (f) inorganic potassium salts are used for the electrolytic production of used in step (g) inorganic acid.

Optional step (j)

Optionally, the inorganic potassium salts respectively obtained according to step (f3) can be further purified, for example by single or multiple recrystallization by means of a suitable solvent such as water and / or by washing with water or a salt solution such as a sodium salt solution. Such recrystallization and / or washing represents an optional step (j).

Further optional steps

Preference is given to carrying out step (e), preferably by adding at least one inorganic acid to the mixture (C) obtained after step (c) or to the concentrate (D1) obtained after step (d) and to the permeate obtained after step (d) ( D2) each set a pH <4, in order to convert optionally contained bicarbonate and / or carbonate into carbon dioxide.

Preferably, the inorganic acid used for this purpose is selected from the group consisting of hydrochloric acid, sulfuric acid and mixtures thereof. The inorganic potassium salts obtainable after step (e) and contained in the concentrates (E1-K) and / or (E2-K) and / or the inorganic potassium salts obtainable after step (f) are preferably used for the electrolytic production of those used in this step used inorganic acids.

Preferably, no alkaline sodium salt (from the outside) is fed into any step of the process according to the invention, in particular no such alkaline sodium salt as sodium hydroxide is used in step (b).

The following examples and comparative examples serve to illustrate the invention, but are not to be construed as limiting.

Example and Comparative Example 1. Comparative example Pre-purification steps (a), (b) and (c)

1 m ^{3 of} pig manure (containing 6% by weight of solids, determination of the solids content according to EN 12880) are subjected to a conventional pre-cleaning. The pig manure is first centrifuged to separate the solids contained therein. The resulting centrifugate is alkalized with 10 L aqueous NaOH (40%) and the resulting mixture is heated to 60 ° C with stirring. Subsequently, the mixture is subjected to a conventional stripping step in a conventional stripping plant to remove ammonia contained therein. The stripping system has a diameter of 10 cm. The packing is designed to achieve a throughput of 40 L / h of fluid. The air flow rate is 4 m ³ / h. The ammonia-containing gas is cooled to remove ammonia-rich water. The condensed water is separated off and the remaining ammonia in the air is washed out with sulfuric acid. Subsequently, 7 L / m ³ aqueous sulfuric acid and are added and subjected to the resulting mixture of an aerobic biological treatment. The aerobic biological treatment is operated as a membrane bioreactor. Part of this biological treatment is the implementation of ultrafiltration. The permeate of the ultrafiltration is subsequently recycled in step (d).

Step (d)

The resulting ultrafiltration permeate is subsequently subjected to nanofiltration. This is carried out at 24 ° C permeate temperature and a pH of about pH 7. As a nanofiltration membrane, a commercially available nanofiltration membrane (NF membrane) is used, which is substantially permeable to monovalent ions such as chloride ions, sodium ions and potassium ions, but substantially impermeable to divalent ions such as sulfate ions. Due to the Donnan effect, the permeability is further increased, especially for chloride ions. The separation limit of the NF membrane used is <200 daltons. The NF membrane is a biofilm-resistant NF membrane (BFR-NF membrane). The NF membrane is based on polysulfones as a carrier material, which is coated with a polyamide. A nanofiltration system is used, which consists of three units per module, ie per pressure tube. Each unit is about 1 m long and has a diameter of about 20 cm. Each unit has an effective area of approx. 35 m ² . After the nanofiltration, a concentrate (D1) and a permeate (D2) are obtained. About 230 L of concentrate (D1) and about 760 L of permeate (D2) are obtained. The permeate (D2) contains about 75% of the chloride ions contained in the ultrafiltration permeates used. The concentrate (D1) contains about 90% of the sulfate ions contained in the ultrafiltration permeate used. The concentrate (D1) and the permeate (D2) each contain about 50% of the sodium and potassium ions contained in the ultrafiltration permeates used.

Steps)

Both the concentrate (D1) obtained after step (d) and the permeate (D2) obtained after step (d) are subjected to electrodialysis. Electrodialysis of (D1) gives the concentrate (E1-K) and the diluate (E1-D) and electrodialysis of (D2) gives the concentrate (E2-K) and the diluate (E2-D). The concentrate (E1-K) is enriched by electrodialysis by about a factor of 3.5. The permeate (D2) obtained after step (d) is subjected to reverse osmosis concentration prior to electrodialysis. The concentrate then obtained (E2-K) is enriched by electrodialysis by about a factor of 12.0.

Step (f)

When concentrating the concentrate (E1-K) (by heating to its boiling point) precipitate small amounts of sodium chloride, which can be separated in the heat at the boiling point until the loss of potassium salts is observed in addition by ICP-OES. It is then filtered off and the resulting filtrate is cooled to 20 ° C, wherein a salt mixture of sodium sulfate, potassium sulfate, potassium chloride and glaserite precipitates. The high levels of sodium contained therein, in particular due to the high amount of sodium sulfate, that is salt mixture obtained is not useful as a fertilizer. When concentrating the concentrate (E2-K) (by heating to its boiling point) precipitates sodium chloride, which can be separated in the heat at the boiling point until the loss of potassium chloride is additionally observed by ICP-OES. It is then filtered off and the resulting filtrate is cooled, whereby potassium chloride precipitates and can be obtained.

The process is a continuous process. To alkalize the mixture used before stripping, new sodium hydroxide is added continuously.

Second example

The same process is carried out comprising the steps (a) to (e) starting from 1 m ^{3 of} the same pig manure as described for the comparative example under point 1 . However, no sodium hydroxide is added before stripping, but 50 L of an aqueous mixture of potassium hydroxide and sodium hydroxide is added, which has a potassium to sodium ion ratio of about 4: 1. This aqueous mixture is obtained by electrolysis of the concentrates (E1-K) and / or (E2-K) obtained according to step (e), so that - after a single addition of such a mixture - the total salt load is not further increased The electrodialysis concentrate (E1-K) has a ratio of potassium to sodium ions of about 4: 1. An analogous ratio also shows the concentrate (E2-K) obtained after conducting the electrodialysis. Each of the concentrates (E1-K) and (E2-K) obtained after step (e) can thus be converted by membrane electrolysis to a mixture of sodium hydroxide and potassium hydroxide, which in turn is used continuously to alkalize the mixture used before stripping (instead the exclusive use of sodium hydroxide as in Comparative Example 1 ). In addition, a mixture of sulfuric acid and hydrochloric acid is used instead of sulfuric acid before the biological treatment.

The obtained after performing the electrodialysis concentrate (E1-K) also has a ratio of chloride to sulfate ions of about 1: 2 on. The concentrates (E1-K) obtained after step (e) can thus be converted into a membrane by means of membrane electrolysis Mixture of hydrochloric acid and sulfuric acid are reacted, which in turn is then used continuously to neutralize the mixture used before the biological treatment (instead of the exclusive use of sulfuric acid as in Comparative Example 1 ).

Each of the concentrates (E1-K) and / or (E2-K) obtained after step (e) can also be converted by membrane electrolysis to a mixture of sodium hydroxide and potassium hydroxide, which in turn is used continuously to alkalize the mixture used before stripping (instead of the exclusive use of sodium hydroxide as in Comparative Example 1 ).

Step (f)

Concentration of the concentrate (E1-K) (by heating to its boiling point) may reduce the volume to 70% of its original volume by adding small amounts of sodium chloride (about 2%, based on total potassium, sodium, chloride and sulfate ions contained in (E1-K) were obtained and separated in the heat and until the addition of ICP-OES in addition to the loss of potassium salts is observed (step (f1)). Then the small amounts of sodium chloride are filtered off (step (f2)) and the resulting filtrate is further concentrated by heating to its boiling point, whereby glaserite precipitates (step (f3)). The yield of glaserite is about 75% (based on the total amount of potassium, sodium, chloride and sulfate ions contained in (E1-K)). The purity of the Glaserits is about 85%, which can be further increased for example by washing with water. Humic acid and humic acid-like compounds (impurities (ii)) can be precipitated from the diluate (E1-D) obtained after step (e) by addition of acid. The resulting glaserite can be used as a fertilizer due to the low sodium content.

Concentration of the concentrate (E2-K) (by heating to its boiling point) causes sodium chloride to precipitate (about 20%, based on the total amount of potassium, sodium, chloride and sulfate ions present in (E2-K) contained), which can be separated in the heat at the boiling point until the loss of potassium salts is observed in addition by means of ICP-OES (step (f1)). It is then filtered off (step (f2)) and the resulting filtrate is cooled to 20 ° C, wherein Potassium chloride precipitates and can be obtained (step (f3)). The yield of potassium chloride is about 77% (based on the total amount of potassium, sodium, chloride and sulfate ions contained in (E2-K)). The purity of potassium chloride is about 80%, which can be further increased, for example by washing with water.

The process is a continuous process. To alkalize the mixture used before stripping, the abovementioned mixture of sodium hydroxide and potassium hydroxide (40% strength), which has a ratio of potassium to sodium ions of about 4: 1, is continuously added. As mentioned above, this is obtained from a reaction of the concentrates (E1-K) and / or (E2-K) membrane electrolysis obtained according to step (e), so that overall new alkalis need not be continuously added to the alkalization (which increase the salt load) and also can be dispensed with the exclusive use of unwanted sodium hydroxide solution.

3 . Experimental results

Comparative example to 1. Amount of ions after carrying out individual process steps ions Centrifugation (a) after centrifugation and before alkalization Ultrafiltration permeate after pre-cleaning Concentrate (D1) after nanofiltration Concentrate (E1-K) after electrodialysis Potassium ions [g / L] 2.0 2.0 6.0 20.0 Sodium ions [g / L] 0.5 3.8 12.0 44.0 Chloride ions [g / L] 1.1 1.1 2.1 7.8 Sulfate ions [g / L] 2.6 7.5 29.0 95.0

Example for 2. - Amount of ions after carrying out individual process steps ions Centrifugation (a) after centrifugation and before alkalization Ultrafiltration permeate after pre-cleaning Concentrate (D1) after nanofiltration Concentrate (E1-K) after electrodialysis Potassium ions [g / L] 2.08 3.99 13.14 44.69 Sodium ions [g / L] 0.63 1.10 3.38 11,50 Chloride ions [g / L] 1.77 2.34 4.86 17.69 Sulfate ions [g / L] 2.35 3.55 13.78 38.59

Claims (15)

A process for recovering inorganic potassium salts from at least one at least partially anaerobic and / or aerobic treatment substrate comprising at least steps (a), (b), (c), (d), (e) and (f), namely
a pre-purification comprising steps (a), (b) and (c): (a) separating solids contained in the at least one substrate which is at least partially accessible to anaerobic and / or aerobic treatment, (b) strip removal of nitrogen-containing compounds contained in the at least one substrate which is at least partially accessible to anaerobic and / or aerobic treatment, wherein the pH of the substrate before conducting the stripping by adding at least one alkaline inorganic potassium salt to a pH > 9 is set and (c) anaerobic and / or aerobic treatment of the substrate which is at least partially accessible to an anaerobic and / or aerobic treatment,
wherein the sequence of steps (a), (b) and (c) is variable and after carrying out these steps (a), (b) and (c) a prepurified mixture (X) is obtained, (d) treating the prepurified mixture (X) obtained after carrying out the prepurification steps (a), (b) and (c) by nanofiltration to obtain a concentrate (D1) and a permeate (D2), (e) treating by electrodialysis the concentrate (D1) obtained after step (d) to obtain a concentrate (E1-K) and a diluate (E1-D) and treating the permeate (D2) obtained after step (d) by electrodialysis to obtain a concentrate (E2-K) and a diluate (E2-D), and (f) treating the concentrates (E1-K) and (E2-K) obtained after step (e) after at least partial concentration of the concentrates in each case independently by crystallization, wherein step (f) is in each case optionally repeated one or more times, Recovery of each of the dissolved inorganic potassium salts contained in the concentrates (E1-K) and (E2-K), which are different from the alkaline inorganic potassium salts used in step (b), characterized in that the inorganic potassium salts obtainable after step (e) and contained in the concentrates (E1-K) and / or (E2-K) and / or the respectively available after step (f) inorganic potassium salts for the electrolytic production of the in Step (b) used alkaline inorganic potassium salts. The method according to claim 1, characterized in that the at least one, anaerobic and / or aerobic treatment at least partially accessible substrate is selected from the group consisting of sewage sludge, manure, residues from biogas plants and mixtures thereof. The method according to claim 1 or 2, characterized in that it is a continuous process. The method according to any one of the preceding claims, characterized in that the alkaline inorganic potassium salts used in step (c) are selected from the group consisting of potassium hydroxide, potassium carbonate, potassium bicarbonate and mixtures thereof. The method according to one of the preceding claims, characterized in that for the implementation according to step (d) at least one nanofiltration membrane is used, which is substantially permeable to monovalent chloride ions, but substantially impermeable to divalent sulfate ions. The method according to one of the preceding claims, characterized in that step (d) is carried out by adding at least one inorganic acid at a pH in the range of 6 to 8, wherein the at least one inorganic acid used for this purpose is selected from the group consisting from hydrochloric acid, sulfuric acid and mixtures thereof, and wherein the dissolved inorganic potassium salts obtainable after step (e) and contained in the concentrates (E1-K) and (E2-K) and / or the in each case obtainable after step (f) inorganic potassium salts electrolytic production of the inorganic acids used in this step. The method according to one of the preceding claims, characterized in that
in the concentrate (E1-K) obtained after step (e), essentially sulphate ions are contained as anions and potassium ions and sodium ions as cations, and
in the concentrate (E2-K) obtained after step (e), essentially chloride ions are contained as anions, and potassium ions and sodium ions are present as cations. The method according to one of the preceding claims, characterized in that the crystallization according to step (f) in each case comprises at least three stages (f1), (f2) and (f3), (f1) concentrating the concentrates (E1-K) and (E2-K) obtained after step (e) each independently at a temperature in the range from 75 ° C to the boiling point of the concentrates (E1-K) and (E2-K ), wherein inorganic sodium salt contained in (E2-K) at least partially precipitates during the performance of (f1) at that temperature and inorganic sodium salt contained in (E1-K) during the execution of (f1), if appropriate, at least partially precipitates at this temperature, and wherein the step (f1) is carried out until the respective inorganic potassium salts contained therein until this time begin to precipitate at this temperature, (f2) filtering off the inorganic sodium salts which have precipitated at least partially after the reaction of step (f1) from (E2-K) and, if appropriate, filtering off the inorganic sodium salts which may be at least partially precipitated after carrying out step (f1) (E1-K) at the abovementioned temperature according to step (f1) in each case independently of one another to give at least partially precipitated inorganic sodium salts and of the filtrate (E2-KF) and of the concentrated concentrate (E1-K) or of (E1-K) at least partially precipitated from (E2-K) inorganic sodium salts and filtrate (E1-KF), (f3) cooling the filtrate (E2-KF) and filtering off then precipitated inorganic potassium salts and concentration of the filtrate (E1-KF) or further concentration of the already concentrated concentrate (E1-K), each to obtain the then from (E1-KF ) or from the already concentrated concentrate (E1-K) precipitated inorganic potassium salts. The method according to claim 8, characterized in that after step (f3) from the filtrate (E1-KF) or from the concentrated concentrate (E1-K) aphthitalite (K ₃ Na (SO ₄ ) ₂ ) and from the filtrate (E2 -KF) potassium chloride is obtained. The process according to any one of the preceding claims, characterized in that the permeate (D2) obtained after step (d) is subjected to a step (d-1) before carrying out the electrodialysis according to step (e) (d-1) Treating the permeate (D2) by reverse osmosis to obtain a concentrate (D2-K) and a permeate (D2-P), wherein the concentrate (D2-K) in the subsequent step (e) instead of the permeate (D2 ) is used. The process according to any of the preceding claims, characterized in that the inorganic potassium salts obtainable after step (e) and contained in the concentrates (E1-K) and / or (E2-K) and / or those respectively obtainable after step (f) inorganic potassium salts are used by membrane electrolysis for the preparation of the alkaline inorganic potassium salts used in step (b). The method according to one of the preceding claims, characterized in that it comprises at least the further steps (g), (h) and (i), namely (g) treating the diluate (E1-D) obtained after step (e) by means of reverse osmosis or nanofiltration to obtain the concentrate (E1-DK) and the permeate (E1-DP), (h) adjusting the pH of the concentrate (E1-DK) by adding at least one inorganic acid to a pH ≤4 and filtering off the then precipitated solids to obtain the filtrate (E1-DKF) and (i) feeding the filtrate (E1-DKF) obtained after step (h) to the concentrate (D1) obtained after step (d). The process according to claim 12, characterized in that the inorganic acid used in step (h) is selected from the group consisting of hydrochloric acid, sulfuric acid and mixtures thereof, and that obtained in step (e) and in the concentrates (E1-K ) and / or (E2-K) contained inorganic potassium salts and / or after step (f) each available inorganic potassium salts for electrolytic Production of inorganic acid used in step (h) are used. The method according to one of the preceding claims, characterized in that prior to performing step (e) by adding at least one inorganic acid to the mixture (C) obtained after step (c) or to the concentrate (D1) obtained after step (d) and to after step (d) obtained permeate (D2) in each case a pH <4 is set to convert optionally contained bicarbonate and / or carbonate into carbon dioxide. The method according to claim 14, characterized in that the at least one inorganic acid used is selected from the group consisting of hydrochloric acid, sulfuric acid and mixtures thereof, and that available in step (e) and in the concentrates (E1-K) and / or (E2-K) contained inorganic potassium salts and / or the respectively obtainable after step (f) inorganic potassium salts for the electrolytic production of these at least one inorganic acid.